If you need to enlarge the frame of a camera without altering the perspective of your original composition, try this:

Divide the original width of your frame by the new width, which in my case was 1200/1600 which gives 0.75. Then multiply your camera focal length by this ratio to find what the new focal length should be. For the example I'm using my original focal length was 35mm, which multiplied by 0.75 is 26.25mm.

Set your camera to this new focal length, change the format accordingly and you have effectively added width to your composition.